It is known to attenuate the intensity of electromagnetic beams by applying an aperture or the like in the path thereof when a detector of the beam becomes saturated. This is a straight forward practice where a beam is collimated. However, where a beam is focused, and is caused to impinge on a sample at oblique angles-of-incidence, the practice becomes more complicated. This is because a focused beam arrives at a sample over a range of angles-of-incidence (AOI), with components passing through a focusing element centrally being at a nominal (AOI) and components thereof which pass through the focusing element laterally being at greater or lesser (AOI's). It is noted that there is also a Plane-of-Incidence (POI) associated with a beam's orientation with respect to a sample which presents similar concerns, and can be handled by similar techniques.
U.S. Pat. No. 7,671,989 to Liphardt et al. described applying an aperture to attenuate beam intensity in such a focused beam situation requires that the aperture be positioned so as to preserve the central component, and substantially equal amounts of the greater and lesser (AOI's) to maintain data affecting characteristics in the attenuated beam similar to those in the unattenuated beam.
It is disclosed that the use of aperture elements in reflectometors and ellipsometers and the like is well known in the art. For instance, a Patent to Liphardt et al., U.S. Pat. No. 7,336,361 discloses an ellipsometer system, in FIG. 1b thereof, with five apertures in the pathway of an electromagnetic beam. U.S. Pat. No. 7,554,662 provides, in FIG. 1a6 thereof, a relevant additional Aperture (NAP) prior to a Detector (DET). Said Patents variously show use of a beam of electromagnetic radiation onto a sample, as is the case in the present invention.
Representative Patents which disclose Apertures in an ellipsometer or the like, provided a Patent to Norton, U.S. Pat. No. 7,145,654. The system therein is described as utilizing a beam focused onto the end of an optical fiber, such that the angular range of the probe beam is less than a natural numerical aperture of an optical fiber. The purpose of the Norton invention is to selectively attenuate and reduce the presence of secondary maxima falling outside a measurement spot on a sample. Minimizing said secondary maxima can improve the amount of light measured by a detector that is reflected from inside a measurement spot. Said 654 Patent is included herein by reference.
A U.S. Pat. No. 5,517,312 to Finarov describes application of a scattered light reducing system at the entry to a Detector in a Rotating Analyzer or Rotating Polarizer Ellipsometer System, which scattered light reducing system consists of two lenses with a hole containing diaphram located midway therebetween, and at the focal lengths of said lenses. Said scattered light reducing system is present after a sample system and processes electromagnetic radiation after it interacts with said sample system. The pin-hole is described as serving to reduce scattered light and providing high spatial resolution.
Another Patent identified is that to Campbell et al., U.S. Pat. No. 5,148,323. Said 323 Patent describes a Spatial Filter in which a pinhole is located other than at the focal length of a converging lens.
U.S. Pat. No. 3,905,675 to McCraken describes a Spatial Filter containing system which enables observation of a weak source of electromagnetic radiation in the presence of strong sources thereof.
U.S. Pat. No. 5,684,642 to Zumoto et al., describes an optical transmission system for use in fashioning an electromagnetic beam for use in machining materials which combines a Spatial Filter and an Optical Fiber.
U.S. Pat. No. 4,877,960 to Messerschmidt et al. is identified as it describes masking energy from outside the target area in a microscope having dual remote image masking.
Continuing, Spectroscopic Ellipsometer Systems are also known in the art. Application a Spatial Filters near a Detector, in the context of Rotating Polarizer and Rotating Analyzer Ellipsometer Systems has been reported, (see U.S. Pat. No. 5,517,312 to Finerov). It is noted, that application of Spatial Filters in Rotating Compensator Ellipsometer Systems, such as the Rotating Compensator Ellipsometer System Claimed in co-owned U.S. Pat. No. 5,872,630.
For general reference, a Patent to Dill et al., U.S. Pat. No. 4,053,232 is disclosed as it describes a Rotating-Compensator Ellipsometer System which operates utilizing monochromatic light. Further, a Patent to Aspnes et al., U.S. Pat. No. 5,877,859 is disclosed as it describes a Broadband Spectroscopic Rotating Compensator Ellipsometer System wherein the Utility is derived from selecting a wavelength range and compensator so that at least one wavelength in said wavelength range has a retardation imposed of between 135 and 225 degrees, and another wavelength in said wavelength range has a retardation imposed which is outside that retardation range. Further Patents of general interest of which the Inventors are aware include those to Woollam et al, U.S. Pat. No. 5,373,359, Patent to Johs et al. U.S. Pat. No. 5,666,201 and Patent to Green et al., U.S. Pat. No. 5,521,706, and Patent to Johs et al., U.S. Pat. No. 5,504,582 are disclosed for general information as they pertain to ellipsometer systems. A Patent to He et al., U.S. Pat. No. 5,963,327 is also disclosed as it describes a laterally compact ellipsometer system which enables providing a focused polarized beam of electromagnetic radiation at an oblique angle-of-incidence, and optionally plane-of-incidence to a sample system in a small spot area.
Patents to Piwonka-Corle, U.S. Pat. Nos. 5,608,526, 5,910,842 and 6,734,967 describe focused beam spectroscopic ellipsometer systems which include means for selecting ranges of angles of incidence reflecting from a sample. Additionally, Patents to Gold, U.S. Pat. No. 5,042,951 and Spanier, U.S. Pat. No. 5,166,752 are disclosed for a similar reason. The present invention differs as no specific guidance as to how to select a range of wavelengths around a nominal angle of incidence to preserve information in a reflected and monitored beam is taught in said Patents.
It is also of interest to note that a computer search for Patents which include both the terms “focused beam” and “aperture” provided only two Patents, namely, U.S. Pat. No. 5,159,412 to Willenborg et al. and U.S. Pat. No. 6,690,473 to Stanke et al., neither of which is particularly relevant to the present invention. Further, U.S. Pat. Nos. 5,910,842, 6,734,967 to Piwonka-Corle et al. are disclosed as they describe use of focused beams in ellipsometry.
Further, as disclosed in Co-pending application Ser. No. 14/545,713, it is known that focusing elements, such as refractive lenses and lens systems, cause both diffraction and aberration to occur in a beam of electromagnetic radiation with which is interacts. It is also known that when the effective diameter of a beam of electromagnetic radiation which impinges on a focusing element is adjusted, the effects of diffraction and of aberration are affected oppositely. That is, as the beam cross-sectional area is increased, the effects of diffraction decrease, but the effects of aberration increase. This leads to a realization that, for each wavelength in the beam, there should be a beam cross-sectional area such that the focusing lens performs “optimally”. That is, there exists a cross-section area such that increase or decrease in cross-sectional area will cause combined diffraction or aberration to become worse, (ie. cause lens performance to be worse). Patents identified in said 713 Parent Application are: U.S. Pat. Nos. 7,239,391; 7,295,313; 6,940,595; and 6,636,309. And, said 713 Parent Application also disclosed Patents that use approaches such as apodizing filters, spatial filters, graded lens etc., to improving imaging performance in metrology systems by adjusting the index of lens material index are:                U.S. Pat. No. 6,738,138 to Wei;        U.S. Pat. No. 7,050,162.        
Finally, two Patents to Liphardt, U.S. Pat. Nos. 8,351,036 and 8,749,785 are identified as they disclose wavelength dependent materials applied in aperturing.
Even in view of the known art, need remains for a system and method of its use which enables maintenance of beam spot size on a sample, and information content on a focused beam of electromagnetic electromagnetic radiation when the intensity thereof is attenuated by application of an apetrue-like element.